Approximately 250 million years ago, almost 90 percent of the planets species
disappeared, marking the largest mass extinction in Earths history. For
years scientists have debated what happened at the Permian-Triassic (P-Tr) boundary.
New evidence from geochronological data on Siberian basalts published by Mark
Reichow of the University of Leicester and colleagues in the June 7 Science
points to volcanism.

This research is not the first to charge volcanism with mass destruction. However,
what is new about Reichow and his teams work is that they record the timing
and volume of the largest known volcanic province, the Siberian Traps. Reichows
team estimates the province is twice as large as previously thought giving it
an area equal to half the size of Australia. They suggest that because the Traps
are so large, they provide compelling evidence that volcanism caused climate perturbation
and in turn the P-Tr extinctions.

This is a type example of a trend within flood basalt research. Once geologists
recognize coherent provinces in terms of geochemistry and ages, new discoveries
keep showing them as bigger and bigger. However, this is by far the biggest and
most significant, says Paul Renne, a geologist at the Berkeley Geochronology
Center.

Approximately a dozen large igneous provinces (LIPs), such as the Siberian Traps,
have occurred in the last 250 million years. LIPs are suspects in the case of
mass extinction because they tend to coincide with these events. Volcanologists
believe that LIPs spewed out magma, particulates and volcanic gases, such as sulfur
dioxide and carbon dioxide, for periods of 1 to 3 million years and at rates higher
than seen anywhere on Earth today.

Scientists widely accept the view that present day volcanic activity, such as
on Iceland, shows that volcanic gases modify the climate. How much gas erupts
will regulate how much the climate and subsequently ecosystems are destabilized,
Renne explains.

We know that the killing mechanisms for the P-Tr extinction seem to be related
to global warming  oceanic anoxia and changes in terrestrial ecosystems.
Our discovery that the Siberian Traps are much larger than previously realized
fits in with this scenario very well  double the size of the Traps and you
double the volcanic carbon dioxide output, says Andrew Saunders, co-author
and also of the University of Leicester.

The basalt samples Reichow and his team used are from the Western Siberian Basin.
Although this basin lies adjacent to and west of the previously known extent of
the Siberian Traps, it lacks basalt exposures, which is why it was not considered
part of the province. Drill cores reaching to depths of 2 to 3 kilometers revealed
basalts that underlie the exposed sedimentary rocks. Some had speculations that
these basalts were related to those of the Siberian Traps, but without accurate
age dating, researchers could not confirm a connection. Reichow and his teams
isotopic dating revealed ages coincident with the Siberian flood basalts. Major
and trace element geochemistry also indicated the basalts had a source similar
to the Trap basalts.

The doubling of the Siberian Flood Basalt Province might explain why the P-Tr
boundary extinctions were also the largest in Earths history. However some
scientists believe that, while volcanism may play a part, it may not be the primary
cause for these extinctions. Instead they favor a scenario of many catastrophes,
including a meteorite impact. For an extinction to occur, the evidence 
specifically paleontological  suggests it must be more complex than from
a single event, says Luann Becker, a geologist from the University of California-Santa
Barbara and leading proponent for meteorite impact as a cause of the P-Tr extinction
(Geotimes, April 2001).

Few scientists dispute that an impact was associated with the Cretaceous-Tertiary
(K-T) boundary extinctions and the demise of dinosaurs 65 million years ago. Also
well recognized at the K-T is the presence of another LIP, the Deccan Traps in
India. Scientists such as Becker hypothesize a link among all three  volcanoes,
meteorite impacts and extinction events  with meteorite impacts being the
primary cause. Becker suggests that when a meteorite crashes into Earth, it causes
perturbations not only in the climate, but also in the solid earth, triggering
volcanism that in turn compounds climate change.

Although, Becker adds, there is no smoking gun for an impact at the P-Tr boundary
as at the K-T boundary, she believes that the presence of cosmic tracers points
to a potential impact at the P-Tr boundary. There is work on the Triassic-Jurassic
boundary which also suggests there could be an extraterrestrial impact at this
time. This makes three extinction events in the history of Earths life associated
with volcanism and potentially with an impact. These are significant catastrophes
and the question is whether there is a causal link between all these events.
Other scientists remain skeptical. The causal links themselves are rather
complex, poorly understood and very difficult to prove, Saunders says. Renne
expresses similar doubts in reference to the current debate on whether cosmic
tracer signatures are conclusively observed in P-Tr boundary sediments.

One way to distinguish between a meteorite impact and volcanism as the primary
cause for extinctions is to study extinction rates. Impacts cause rapid (tens
of thousands of years) extinction rates. In contrast, extinctions caused by volcanoes
alone take millions of years. As Saunders explains, estimates for the duration
of the P-Tr extinctions vary widely.

Despite the debate, Renne says, Reichow and colleagues work strengthens
the causal link between the Siberian Traps and the P-Tr extinctions. The
discussion of whether or not a meteorite impact occurred is completely separate
from, and unaffected by, our research, Saunders says. The jury is
still out on this one.
Salma Monani